Can people using different computers edit the same .blend file at once?
Currently this isn't possible and would require major changes to Blender.

Typically larger projects divide assets into many files so people can work on different parts simultaneously, this can be done so assets can be re-used in many places using Blender's library linking system.

Its also common to use a version control system, such as Git, Subversion, Mercurial... (or many others), to manage files for larger projects.

For reference, Mango Open Movie used this structure.